1. Field of the Invention
The invention herein relates to the removal from the environment of polychlorinated biphenyl compounds (PCBs). More particularly it relates to environmentally safe compositions and methods which degrade PCBs.
2. Description of the Prior Art
Polychlorinated biphenyls are haloaromatic compounds of exceptional chemical stability. Environmental and toxicological problems caused by the use of PCBs resulted in a restriction on their production under the Toxic Substances Control Act of 1976 and a complete ban of their manufacture by the EPA in 1979. Past negligent disposal practices have caused substantial PCB contamination of soils and surface water sediments, so that at least 15% of the PCBs manufactured in the United States now remains in the environment as highly recalcitrant contaminants. Among the acute toxicological endpoints observed following exposure to PCBs from industrial accidents are chloracne (a skin disease) and hepatotoxicity (liver damage). Of greater concern are the mutagenic and carcinogenic properties of PCBs and their suspected role in the reproductive failure of wildlife species. This role in the reproductive failure is aggravated by the lipophilic nature of PCBs and their tendency to bioaccumulate in the food chain. Other concerns with PCBs include the occurrence of impurities such as polychlorinated dibenzofurans, which are extremely toxic chemicals.
The Resource Conservation and Recovery Act of 1976 legislated a "cradle-to-grave" approach to the management of hazardous wastes, making the manufacturer and user liable for their safe disposal. The Hazardous and Solid Waste Amendment of 1984 further implemented strict regulations on the land disposal of wastes containing PCBs, dioxins, and other halogenated chemicals. Incineration is the principal PCB disposal method, but the long-range atmospheric transport of particulate-associated PCBs has resulted in the distribution of PCBs across the globe.
The current best technology for remediation of PCB-contaminated soils is excavation, followed by landfill, storage, or incineration. However, the removal of PCB-contaminated soils is a tremendous expense because of the large number of contaminated sites throughout the nation. There are also many large land areas containing low, but significant, levels of PCBs, which would be impractical to treat by conventional technology. Therefore, low cost, in situ technologies for removal of PCBs from contaminated land areas are in high demand.
Because microbiological degradation of toxic waste does not involve the use of chemical reagents which might themselves be toxic, does not produce large amounts of toxic waste, and is comparatively low cost, microbiological degradation is a better method of disposing of toxic waste than incineration or removal. Specifically, bioremediation of soils by PCB-degrader microbes is a promising, low cost technology for in situ treatment of PCB-contaminated soils. However, there are many problems unique to PCB degradation that have slowed the development of an effective approach for degradation of these compounds.
Natural environments contain indigenous microbes having differing capabilities for metabolizing PCB contaminants. Some microbes can only metabolize PCBs at a slow rate, such that there is only a minimal decrease in the concentration of PCBs persisting in the soil over time. Still other indigenous microbes are not well enough understood to be used in any commercially practical manner.
The initial step in the metabolism of PCBs to innocuous metabolic products is cometabolic, meaning that the organism responsible for the PCB degradation gains no energy from the process. See, R. S. Horvath, Bacteriological Reviews, 36:146 (1972). Therefore, the PCB degradation must be sustained by an alternative compound which serves as the growth substrate. Cometabolism is understood by those skilled in the art as presenting a barrier to the effective remediation of PCB-contaminated environments. PCB degradation occurs only if the requisite enzymes have been induced by growth on a suitable substrate. It is understood by those skilled in the art that those microbes that are capable of degrading PCBs will not do so unless and until their bph genes are induced to produce the enzymes required for PCB biodegradation. It is also understood by those skilled in the art that an inducer of bph genes has some aspect of its chemical structure that is analogous to PCBs.
Most current methods for microbial degradation of toxic materials require the discovery of a particular microbe that will metabolize the toxic material, converting it to innocuous metabolic products. Finding microbes which can efficiently and safely metabolize toxic wastes is a complex procedure involving many arduous steps and requiring a significant expenditure of time. One such method is taught in U.S. Pat. No. 4,493,895 (1985) to Colaruotolo et al., wherein is described a method for the microbial degradation of organic wastes into innocuous materials. This process involves (1) collecting a sample of material from the site contaminated with toxic wastes; (2) enriching the microbes found living in the sample; (3) separating the strains of microbes from each other; (4) purifying the strains that can degrade the toxic substance; (5) applying the strain to the contaminated area; and (6) monitoring of degradation of the toxic substance at the contaminated area. Another method taught in U.S. Pat. No. 4,511,657 (1985) to Colaruotolo et al. involves a process of treating chemical waste landfill leachates with activated sludge containing bacteria capable of metabolizing toxic substances present in the leachates. Both of these methods require large amounts of time and effort. It would be desirable if, rather than isolating and characterizing a particular microbe that can degrade PCBs and adding it to the PCB-contaminated environment, indigenous microbes could be induced to accomplish the degradation of PCBs.
Some bacteria have recently been shown to be capable of degrading PCBs in the laboratory. In Microbial Degradation of Organic Compounds, David T. Gibson, Ed., p. 362, Marcel Dekker, Inc., New York (1984), the metabolism of commercial PCB mixtures and biphenyl is discussed, but no commercially practical process for degrading PCBs in a natural environment is described. K. Furukawa et al, Applied And Environmental Microbiology, 46:140 (1983) describe the use of Acinetobacter strains to metabolize commercial mixtures of PCBs and deduce a pathway for PCB metabolism. These references do not disclose a commercially practical process for the decontamination of PCB-contaminated environments. None of the references teaches how non-indigenous microbes specifically adapted for the metabolism of the contaminant can be utilized with microbes indigenous to the environment to accomplish decontamination at a practical rate.
U.S. Pat. No. 4,664,805 (1987) to Focht describes how environments contaminated with toxic halogenated organic compounds can be decontaminated at an accelerated rate by the addition to the contaminated environment of non-indigenous microbes along with analogs of the halogenated contaminant. Among the microbes that can be added to the contaminated soil are Acinetobacter sp. (Furukawa), Strain P6, Pseudomonas putida, Strain UC-R5, and Pseudomonas putida, Strain UC-P2. The specific PCB analogs added to the PCB-contaminated soil is biphenyl (the non-chlorinated analog of PCBs). Biphenyl has the following chemical structure: ##STR1##
In the laboratory, biphenyl can be used successfully as an inducer and growth substrate for PCB degradation. However, biphenyl is subject to environmental regulation as a designated pollutant and cannot be introduced into soils. Consequently, there is a continuing need to identify other compounds that promote PCB bioremediation yet are safe to introduce into the environment.